Problem using reinterpret_cast<> in c++

Question

I am trying to cast a datastream into a struct since the datastream consists of fixed-width messages and each message has fulle defined fixed width fields as well. I was planning on creating a struct and then using reinterpret_cast to cast pointer to the datastream to the struct to get the fields. I made some test code and get weird results. Could any explain why I am getting these or how to correct the code. (the datastream will be binary and alpha numeric mixed but im just testing with strings)

#pragma pack(push,1)
struct Header 
{
    char msgType[1];
    char filler[1];
    char third[1];
    char fourth[1];
};
#pragma pack(pop)

int main(void)
{
    cout << sizeof(Header) << endl;

    char* data = "four";
    Header* header = reinterpret_cast<Header*>(data);
    cout << header->msgType << endl;
    cout << header ->filler << endl;
    cout << header->third << endl;
    cout << header->fourth << endl;
    return 0;
}

The result that are coming up are

4
four
our
ur
r

I think the four, our and ur is printing since it cant find the null terminator. How do I get around the null terminator issue?

Answer 1

You're right about the lack of null terminator. The reason it's printing "ur" again is because you repeated the header->third instead of header->fourth. Instead of "char[1]", why not just declare those variables as "char"?

struct Header 
{
    char msgType;
    char filler;
    char third;
    char fourth;
};

Answer 2

The issue is not reinterpret_cast (although using it is a very bad idea) but in the types of the things in the struct. They should be of type 'char', not of type 'char[1]'.

Answer 3

#pragma pack(push,1)
template<int N>
struct THeader 
{
    char msgType[1+N];
    char filler[1+N];
    char third[1+N];
    char fourth[1+N];
};
typedef THeader<0> Header0;
typedef THeader<1> Header1;  
Header1 Convert(const Header0 & h0) {
   Header1  h1 = {0};
   std::copy(h0.msgType, h0.msgType + sizeof(h0.msgType)/sizeof(h0.msgType[0]), h1.msgType);
   std::copy(h0.filler, h0.filler+ sizeof(h0.filler)/sizeof(h0.filler[0]), h1.filler);
   std::copy(h0.third , h0.third + sizeof(h0.third) /sizeof(h0.third [0]), h1.third);
   std::copy(h0.fourth, h0.fourth+ sizeof(h0.fourth)/sizeof(h0.fourth[0]), h1.fourth);
   return h1;
}
#pragma pack(pop)


int main(void)
{
  cout << sizeof(Header) << endl;
  char* data = "four";
  Header0* header0 = reinterpret_cast<Header*>(data);
  Header1 header = Convert(*header0);
  cout << header.msgType << endl;
  cout << header.filler << endl;
  cout << header.third << endl;
  cout << header.fourth << endl;
  return 0;
}

Answer 4

In order to be able to print an array of chars, and being able to distinguish it from a null-terminated string, you need other operator<< definitions:

template< size_t N >
std::ostream& operator<<( std::ostream& out, char (&array)[N] ) {
     for( size_t i = 0; i != N; ++i ) out << array[i];
     return out;
}

Answer 5

In my experience, using #pragma pack has caused headaches -- partially due to a compiler that doesn't correctly pop, but also due to developers forgetting to pop in one header. One mistake like that and structs end up defined differently depending on which order headers get included in a compilation unit. It's a debugging nightmare.

I try not to do memory overlays for that reason -- you can't trust that your struct is properly aligned with the data you are expecting. Instead, I create structs (or classes) that contain the data from a message in a "native" C++ format. For example, you don't need a "filler" field defined if it's just there for alignment purposes. And perhaps it makes more sense for the type of a field to be int than for it to be char[4]. As soon as possible, translate the datastream into the "native" type.

Answer 6

Assuming you want to keep using an overlayable struct (which is sensible, since it avoids the copy in Alexey's code), you can replace your raw char arrays with a wrapper like the following:

template <int N> struct FixedStr {
    char v[N];
};

template <int N>
std::ostream& operator<<( std::ostream& out, FixedStr const &str) {
    char const *nul = (char const *)memchr(str.v, 0, N);
    int n = (nul == NULL) ? N : nul-str.v;
    out.write(str.v, n);
    return out;
}

Then your generated structures will look like:

struct Header 
{
    FixedStr<1> msgType;
    FixedStr<1> filler;
    FixedStr<1> third;
    FixedStr<40> forty;
};

and your existing code should work fine.

NB. you can add methods to FixedStr if you want (eg, std::string FixedStr::toString()) just don't add virtual methods or inheritance, and it will overlay fine.